Already known from French Pat. No. 2,479,981 and, more particularly, from the second addition thereof, U.S. Pat. No. 2,496,260, is an integral assembly that permits the regrouping, in the same long-distance transmission fiber, of several light beams of different wavelengths carried in as many input fibers. Alternatively, in reverse order, it permits the separation of several light beams of different wavelengths, carried in a single transmission fiber, into several beams directed to different optical fibers.
In such a device, the ends of the input and output fibers are embedded in a first transparent block and terminate in an opening placed in a plane diffraction grating formed on one endface of this first block. The face of the first block carrying the grating is glued to a second block whose opposite endface forms a spherical, or preferably, a parabolic mirror, the focus of which lies in the immediate vicinity of the ends of the fibers.
While such a device is optically quite satisfactory, it is not well-suited for large-scale industrial production as now demanded by the development of teletransmission via fibers.
A first difficulty lies in the production of the first block where, after forming a replicated diffraction grating on it, it is necessary to make a central hole to permit the flush positioning of the ends of the input and output fibers. Now only can this procedure cause local damage to the edges of the diffraction grating around the hole but it is thereafter very difficult to place the fibers into exact relative positions within the block and the hole. A second difficulty derives from the fact that a high degree of stigmatism practically dictates the use of a parabolic mirror rather than a spherical one which leads to increased costs of production.
The invention aims at an optical assembly that is easy to manufacture in mass production and offers high stigmatic quality while using a simple spherical mirror.
The invention is applicable to a multiplexer-demultiplexer of the type in which the light beams of different wavelengths that are to be mixed or separated are carried via optical transmission fibers whose ends are disposed in the immediate vicinity of the focus of a concave mirror associated with a plane diffraction grating.